Determining pulse width for the GTS?

[Uberbots]

im pretty sure that this is a fairly simple topic... but i cant figure it out.
What i am looking at is Kevin's encoder code and trying to determine the interrupt routines i would need to measure a pulse width from the Gear Tooth Sensor. This is kind of important because i need to know direction, and looking at the specs for the GTS i can see that a CW tick has a pulse width of 45 us, and CCW has 90 us.

I know how i would detect a tick, but how would i detect the pulse width?

EDIT: some new realizations by me tell me that the PWM pins can be configured as inputs for measuring signals like this...